BACK ILLUMINATED IMAGING
High-speed imaging of an agricultural spray. Shadowgraphy technique permits near-nozzle break-up region to be observed in detail FATCAM SA5
About the Technique
Back illuminated imaging is a widely used imaging technique. Under the correct conditions it can provide high contrast images of object’s. This is particularly important if you are interested in an objects’ shape or size.
The idea is a simple one – a light is shone towards the camera creating a white image, the subject is then passed between the light source and the camera thus blocking the light path and creating a dark area on the image. If the subject is stationary you can focus your camera onto the objects’ profile and obtain a sharp outline. If the subject is moving, in order to freeze the motion (remove blur) and obtain a crisp outline you will need to have a sufficiently fast shutter (short exposure time).
In extreme cases a strobe light source such as a pulsed laser or LED can be used. (For more detail on when to use a laser please refer to Case Study: Laser Illumination.) Using this technique you will not be able to obtain 3D information about the object shape. If you wish to resolve features on the object surface you can use a combination of front and back lighting. The technique is not limited to solids, it can equally be applied to fluids, multi-phase flows, in fact any subject that will completely or partially block the path of light to the imaging sensor.
Optical Set up: The best results are obtained with a diffuse light source. This can be achieved by passing the light through a light diffusing medium such as a ground glass or opal diffuser plate, alternatively as a low cost solution opaque drawing film works very well. Where it is not possible to position the light behind the subject you can reflect it back to the camera using a white background
- 1. Spray analysis
- 2. Droplet impaction / coalescence
- 3. Particle size and shape
- 4. Bubble formation and growth
- 5. Ballistics
- 6. Welding
Photo: High-speed of shotgun being fired. A combination of front and reflected back-illumination clearly shows both the pellets and a sharp profile of the wad.
High Speed Photography and Photonics
Sidney Ray , Published October 2002 , Hard Cover; 0-8194-4527-4
Published by SPIE–The International Society for Optical Engineering,
P.O. Box 10, Bellingham, Washington, 98227-0010, USA